Currently, the majority of all vehicles driven today use front-end accessory drive electric machines also referred to herein as alternators or starter alternators. These electric machines are typically driven by belt and contain Lundell style rotors, also known as “claw pole” rotors. The rotor provides the electric machine with a magnetic field and rotates within the machine. The rotor includes a coil assembly having a field coil made up of an insulated copper wire or wires wrapped around an electrically insulating bobbin. The bobbin surrounds a steel core, and insulates the field coil from the steel pole pieces which sandwich the field coil to form north and south poles. The magnetic field is generated when the field coil is energized and a current flows through the wires.
One problem with conventional rotors is preventing rotational movement of the field coil within the rotor assembly. The rotor is driven via a belt by the engine of the vehicle. The engine is constantly changing speeds during operation leading to accelerations and decelerations of the rotor speed. Typical vehicles experience acceleration and deceleration rates of approximately 15,000 RPM/sec with transit excursions as high as 30,000 RPM/sec. Movement of the field coil wires leads to a variety of coil failures including wire fatigue fractures, insulation abrasion, and bobbin insulator wear.
Therefore, it is important in the rotor design to prevent the field coil from moving within the rotor assembly. Conventional solutions to this problem include locking features formed into the coil assembly and the pole pieces, as well as the use of epoxy fillers or other adhesives to attach the coil assembly to the pole pieces. For example, projections may be formed into the outside face of the bobbin that mate with indented features in the poles to help lock the bobbin and hence coil assembly in place.
Unfortunately, these locking features remove steel from the pole pieces, leading to higher magnetic saturation in the poles and reducing power density. In addition, the thick locking protrusions created on the bobbin are made of plastic bobbin material that is a poor conductor of heat, preventing good heat transfer from the coil to the cooler poles and leading to an increase in field coil temperature. Likewise, the use of epoxy filler takes up space that could otherwise be filled by the field coil and prevents good heat transfer, both of which decrease the power density of the alternator. In sum, current methods of locking the field coil in position create unwanted performance tradeoffs.
More recent advancements rely on interference between the pole pieces and the coil windings themselves to prevent rotation of the coils. U.S. Pat. No. 6,707,227 discloses such a structure, specifically, the coil interference with axially extending portions of the fingers of the pole pieces. The inner diameter formed by the axial extensions of the pole fingers contact the outer diameter of the coil assembly resulting in deformation of the coils into a zigzag shape as viewed axially and radially. Drawbacks of this design include the requirement for the coil assembly diameter to be matched closely with the inner diameter of the pole fingers, which means that many applications would have either, more wire in the coil than they need, or would require customized pole pieces to accommodate the diameters of smaller coils. Both conditions incur cost penalties, the first for the excess wire and the second for the custom tooling and lower volumes for the customized pole pieces. Additionally, in the condition with the excessive coils the extra wire in the coil adds inertia to the rotor slowing its acceleration and deceleration response times.
Accordingly, there exists a need to provide alternator rotor pole pieces that prevent field coil movement, within the rotor assembly, while allowing for increased heat dissipation and improved cost efficiency of the alternator.